Thursday 30 July 2015

Babibasiliscus alxi: A Casquehead Lizard from the Early Eocene of Wyoming.

Casquehead Lizards, Corytophanidae, are a group of Iguanid Lizards found from the tropical forests of southern Mexico, through Central America to the northwest of South America. They belong to a group of Iguanians, the Pleurodonta, found today almost exclusively in the Americas, with non-American members of the group known only from the islands of Madagascar and Fiji, and studies of the group taking into account relationships between modern species and their biogeographical distribution suggest that they probably originated in the American tropics. However a single fossil specimen of a Casquehead Lizard is known from the Early Eocene of Central Europe, suggesting that their ancient distribution might have been somewhat different from that seen today.

In a paper published in the journal PLoS One on 1 July 2015, Jack Conrad of the Department of Anatomy at the New York Institute of Technology College of Osteopathic Medicine and the Department of Vertebrate Paleontology at the American Museum of Natural History describes a new species of Casquehead Lizard from the late Early Eocene Bridger Formation of Uinta County in Wyoming (about 48 million years old).

The new species is named Babibasiliscus alxi, where ‘Babi-’ means ‘older male cousin’ in the language of the Shoshone people who inhabited the region where the fossil was discovered, and ‘-basiliscus’ (Basiliscus) is the name of a modern genus of Casquehead Lizards, while ‘alxi’ honours John Alexander, who discovered the specimen from which the species was described. The single known specimen comprises a nearly complete skull with articulated jaws plus the first two vertebrae, undistorted or compacted, but with the premaxilla, posterior half of the frontal, parietal, postfrontals, postorbitals, squamosals, supratemporals, supraoccipital, dorsal half of each quadrate, and the anterior tips of both dentaries lost to erosion.

Babibasiliscus alxi: Photographs in (A) right lateral, (B) dorsal, and (C) ventral views. Digital reconstructions derived from High Resolution X-ray Computerised Tomography in (D) left lateral view and (E) transverse section. The vertical red line in (D) indicates the plane of section in (E). Conrad (2015).

A phylogenetic analysis of Casquehead Lizards including Babibasiliscus alxi places it firmly within the modern group, closely related to the living genus Laemanctus. Wyoming is considerably outside the range of modern Casquehead Lizards, with a strongly seasonal climate, typically including heavy winter snowfall. However during the Eocene global climates are known to have been considerably warmer than today, and both Babibasiliscus alxi from Wyoming and Geiseltaliellus maarius from the Eocene of Central Europe are interpreted as having lived in tropical forests not dissimilar from those inhabited by modern Casquehead Lizards.

Phylogeny of Corytophanids and distribution of the group. (A) Time-calibrated phylogeny of the Corytophanid genera with representative illustrations of their skull morphologies. Conrad (2015).

Modern Iguanians are divided into two groups, the Chamaeleontiformes, comprising the Chameleons, Agamas, and their relatives, which are found in the Old World, and the Pleurodonta, comprising the Iguanas, Basilisks, Anoles, Fence Lizards, and their relatives, which are found in the Americas (with the exception of the Opluridae which are found in Madagascar and the genus Brachylophus from Fiji). However the earliest Iguanians are found in Cretaceous deposits dating to before the separation of North America and Eurasia, with the majority of species found in Asia, suggesting this is where the group originated. By the Eocene Iguanians were divided into the modern groups Chamaeleontiformes and Pleurodonta, though with specimens from both groups found on both sides of the Atlantic, and in particular many species of Pleurodonts found in the Old World. It is therefore thought that the modern distribution of the groups reflects a relict distribution shaped by post-Eocene cooling rather than the modern groups having derived from isolated populations after the separation of the continents. Conrad suggests that this interpretation can also be applied to the modern Casquehead Lizards, with the group distributed across tropical forests in what is now the Northern Hemisphere temperate zone during the Eocene (when they were probably excluded from the far hotter tropics), and subsequently having withdrawn to the Neotropics during later Cainozoic cooling.

See also…

Anles are small Iguanid Lizards found in the Southern United States, Mexico, Central America and the Caribbean. Twig Anoles, Norops spp., form a distinct cluster of medium sized Anole Lizard species within the genus Norops. There are currently eight...

Treerunners of the genus Plica are Iguanid Lizards found in South America east of the Andes. They are medium sized, conspicuous Lizards that are active...

Western Europe is thought to have had a diverse Squamate (Snake and Lizard) fauna during the Miocene, but to have largely lost this during the cooling climate of the Plio-Pleistocene, during which...

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